1. Field of the Invention
The invention relates to a method and system for controlling a hybrid electric vehicle powertrain to maintain battery charging power and battery discharging power within predetermined power limits.
2. Background Discussion
In a typical hybrid electric vehicle powertrain for contemporary automotive vehicles, there are at least two power sources. One power source is an engine, such as an internal combustion engine, and a second source is an electric motor-generator system. A battery, which is a part of the motor-generator system, acts as an energy storing medium. The power sources may operate simultaneously to meet a driver demand for torque at the traction wheels of the vehicle. Examples of powertrains of this type are disclosed in U.S. Pat. No. 6,490,511 (Raftari, et al.); U.S. Pat. No. 6,553,287 (Supina et al.); U.S. Pat. No. 6,555,927 (Suzuki et al.); and U.S. Pat. No. 6,688,411 (Boggs et al.).
A vehicle system controller, when the motor-generator system and the engine operate together, will interpret a driver demand for acceleration or deceleration as a demand for battery charge power or discharge power. The vehicle system controller will coordinate the control of the two power sources and determine how much battery power is needed to meet the driver demand and to achieve a specified vehicle performance with respect to fuel economy, exhaust emission quality and drivability. Controlling battery power so that the battery discharge and charge power limits are not exceeded will maintain battery life. The invention can be used with other hybrid electric vehicle powertrains as well.
In a known technique for protecting battery life in a hybrid electric vehicle powertrain, where the vehicle system controller interacts with the motor, the generator and the engine, desired wheel power is clipped. This is done to maintain system power limits and to ensure that a battery discharge limit and a battery charge limit are not violated. Merely clipping the desired wheel power, however, may not always ensure that the battery charge and discharge power limits are not violated because inaccuracies may occur in engine power estimation during steady state operation and during transients. Further, inaccuracies may occur due to generator and motor transients, steady state losses and rate of change of motor torque and generator torque.